1. Field of the Invention
This invention relates to the improvement of a focus-correcting circuit of a display unit using a cathode-ray tube (hereinafter referred to as CRT) used as a CRT display monitor, a television display, and so forth.
2. Description of the Prior Art
Focus-correction signals having the waveforms of so-called parabolic waveforms and periods equal to vertical and horizontal scanning periods, are conventionally used for equalizing the focusing degrees on the periphery and in the center of the surface of a CRT.
FIG. 10 is a circuit diagram showing a conventional focus-correcting circuit for a CRT disclosed in, for example, Published Unexamined Utility Model Application of 105264/89 (Jikkai-Hei 1-105264). In the figure, reference numeral 51 denotes an input terminal, and numeral 52 denotes an output terminal.
The operation of the circuit will be described with reference to accompanying drawings in the following. When a sawtooth wave signal 4 for deflection shown in FIG. 11(a) is input on the input terminal 51 of the circuit of FIG. 10, a signal having a waveform 6 of FIG. 11(b) is obtained on the output terminal 52 of the circuit as a dynamic focusing signal if the amplitude of the sawtooth wave signal 4 is small, and a signal having a waveform 7 of the figure is obtained if the amplitude of the signal 4 is large.
The output voltage of the circuit begins to fall when the AC amplitude of the input signal becomes larger than a certain value, and the diode D1 turns on when the output voltage becomes (V1-Vf) at the point A of time of FIG. 11(b) (where reference letter V1 denotes the DC bias voltage at the input terminal of the integrating circuit of FIG. 10, and letter Vf denotes the forward drop voltage of the diode D1). Consequently, the integrated circuit IC1 operates as only a voltage follower to output the voltage of (V1-Vf). That is to say, while the voltage of the output signal is to be lower than (V1-Vf), the diode D1 turns on to output a constant voltage (V1-Vf) from the circuit. Then, when the voltage of the output signal begins to rise at the point B of time of FIG. 11(b), the diode D1 immediately turns off, and the integrated circuit IC1 operates as an integrating circuit.
The value of the minimum voltage on the output terminal 52 thus fixed at about (V1-Vf) owing to the effect of the diode D1 and the capacitor C2 for integration, and thereby a dynamic focusing signal having a waveform whose peaks is clipped by an amount necessary for clamping operation during fly-back line periods can be obtained.
Since the waste waveforms designated by reference numeral 3 in FIG. 11(b), which are to be output during fly-back line periods originally, are not output but clipped by constructing the circuit as described above, the circuits after the next step can be constructed by using the amplifiers having dynamic ranges smaller than those which are to receive dynamic focusing signals having waveforms not clipped.
The clipping level is determined so that the interval of clipping time is completely shorter than the fly-back line period shown in FIG. 11(a). Therefore, there are no problems as far as the CRT to which the circuit is applied is used to be connected to a specified personal computer or the like which has a fixed format, but if the CRT is used to be connected to an apparatus having the format of so-called multiple scanning or automatic tracking where horizontal scanning frequencies vary widely, as shown in FIG. 12(a) and FIG. 12(b), the clipping level cannot help being determined by being adjusted to the shortest fly-back line period shown in FIG. 12(a).
If the CRT the focus-correcting circuit of which has the clipping level thus determined is used to the apparatus having a longer fly-back line period or a longer time interval as shown in FIG. 12(b), the interval of clipping time is short to increase waste outputs, which decreases the effect of clipping. Because vertical parabolic-waveforms have a fly-back line time much shorter than the scanning time, the aforementioned problem hardly matter.
Since the conventional focus-correcting circuit is constructed as described above, the circuit used in a monitor which can be applied to various personal computers having different horizontal scanning frequencies (or an automatic tracking monitor) is designed to be fitted for the apparatus having the shortest fly-back line period. Therefore, the circuit has a problem that, if the monitor is applied to a computer having longer fly-back line period, a waste amplitude is generated to make it impossible to obtain the expected effect.
The conventional circuit has another problem that the adjustment of the clipping level is not easy, because the clipping is not adjusted to timing, which is desirable, but to levels.